456 CHEMICAL ENGINEERING TABLE 15.—PLATINUM GAUZE, 80-MESH, 0.003-iN. WIDE. TEMPERATURES OBSERVED THROUGH ONE WINDOW BY OPTICAL PYROMETER SIGHTED NORMAL TO SURFACE OF GAUZE, vs. TRUE TEMPERATURE OF GAUZE Observed temperature, degrees Centigrade True temperature, degrees Centigrade Observed temperature, degrees Centigrade True temperature, degrees Centigrade 600 675 850 975 650 730 900 1,035 700 790 950 1,095 750 850 1,000 1,160 800 910 1,050 1,220 f a Window.—It is frequently necessary, especially in the laboratory, i optical pyrometer into a furnace through a window. What correction ipplied to the observed temperatures to take account of the loss of ,ne window? Kanolt has measured the transmission coefficient for a 3f ordinary glass windows at X = 0.65/j, and obtained a mean value of Hence we have 1 1 _ log 0.904 _ nnnnn4fi tf-§-"9^88-------0.0000046 where # is the true absolute temperature of the source and S is the observed absolute temperature. The following table is computed from the above formula: TABLE 16.—CORRECTION TO OBSERVED TEMPERATURES FOR ABSORPTION OF LIGHT BY A SINGLE CLEAN WINDOW Observed temperatures, degrees Centigrade Correction to add, degrees Centigrade Observed temperatures, degrees Centigrade Correction to add, degrees Centigrade 600 3.5 1,600 16.0 800 5.4 1,800 20.0 1,000 8.0 2,000 24.0 1,200 10.0 2,500 36.0 1,400 13.0 3,000 50.0 Flames and Smoke.—The optical pyrometer cannot be used satisfactorily when sighted through flames or smoke. Usually the presence of dense flames increases the temperature reading, and the presence of smoke clouds absorbs so much radiation that the pyrometer may read several hundred degrees low. The optical pyrometer can be used to measure the temperature of the slag in an open-hearth furnace but the flames prove a serious hindrance except during reversals when observations may be taken to advantage. In a cement kiln the dust, smoke and flames all combine to make the observations very untrustworthy. Carbon dioxide, water vapor and other invisible gases produce no effect.